Full Idea
In Locke, the needs of scientific explanation are what determine which qualities are to be taken as primary.
Gist of Idea
Qualities are named as primary if they are needed for scientific explanation
Source
report of John Locke (Essay Conc Human Understanding (2nd Ed) [1694]) by Peter Alexander - Ideas, Qualities and Corpuscles 6
Book Reference
Alexander,Peter: 'Ideas, Qualities and Corpuscles' [CUP 1985], p.142
A Reaction
Not a sharp distinction, but interesting. It must concern 'objective' explanations to cut out the secondary qualities.